Cotton (Gossypium spp.) is the world's most important textile fiber crop and is one of the world's most important oilseed crops. Cotton plants provide a source of human food, livestock feed, and raw material in industry. Cotton seed is pressed for cooking oil and the residual cottonseed oil meal is used for animal feed. Industrial uses of cotton include candle wicks, twine, paper and a multitude of fabric products.
The genus Gossypium is very large, currently containing more than 50 species. Two tetraploid species of Gossypium have spinnable seed fibers called lint. These two species are G. hirsutum (referred to as American Upland cotton) and G. barbadense (referred to as Pima cotton).
The goal of a cotton breeder is to improve a cotton plant's performance and therefore, its economic value by combining various desirable traits into a single plant. Improved performance is manifested in many ways. Higher yields of cotton plants contribute to more lint fiber production, a more profitable agriculture and a lower cost of products for the consumer. Improved plant health increases the yield and quality of the plant and reduces the need for application of protective chemicals. Adapting cotton plants to a wider range of production areas achieves improved yield and vegetative growth. Improved plant uniformity enhances the farmer's ability to mechanically harvest cotton.
Cotton is a dicot plant with perfect flowers, i.e., cotton has male, pollen-producing organs and separate female, pollen receiving organs on the same flower. The cultivated cotton flower bud is surrounded by three triangular bracts forming what is commonly known as squares. The flower contains an open corolla with five petals, a staminal column bearing clusters of stamens and forming a tube that encloses the style. The compound pistil consists of three to five carpels with stigmas protruding above the anthers. The ovary develops into a three- to five-loculed capsule or boll. From seven to nine seeds are set within each loc or locule. On the day preceding anthesis, a twisted corolla emerges from the square. On the day of anthesis, the corolla opens and pollen shedding occurs. The corolla turns red the day following anthesis and later falls from the plant. Pollination occurs with the opening of the anthers and shedding of pollen on the stigma or with the deposit of pollen on the stigma by insects.
Because cotton has both male and female organs on the same flower, cotton breeding techniques take advantage of the plant's ability to be bred by both self-pollination and cross-pollination. Self-pollination occurs when pollen from the male organ is transferred to a female organ on the same flower on the same plant. Self-incompatibility is a form of infertility caused by the failure of cotton plants with normal pollen and ovules to set seed due to some physiological hindrance that prevents fertilization. Self-incompatibility restricts self-pollination and inbreeding and fosters cross-pollination. Cross-pollination occurs when pollen from the male organ on the flower of one plant is transferred to a female organ on the flower on a different plant.
A plant is sib-pollinated (a type of cross-pollination) when individuals within the same family or line are used for pollination (i.e. pollen from a family member plant is transferred to the stigmas of another family member plant). Self-pollination and sib-pollination techniques are traditional forms of inbreeding used to develop new cotton varieties, but other techniques exist to accomplish inbreeding. New cotton varieties are developed by inbreeding heterozygous plants and practicing selection for superior plants for several generations until substantially homozygous plants are obtained. During the inbreeding process with cotton, the vigor of the lines decreases and after a sufficient amount of inbreeding, additional inbreeding merely serves to increase seed of the developed variety. Cotton lines are typically developed for use as varieties once they have been selected to an acceptable degree of uniformity and occasionally they are used in the production of hybrid cotton lines.
Natural, or open pollination, may include both self- and cross-pollination. Such cross pollination is accomplished almost entirely by the bees or other pollinating insects as the pollen is heavy and sticky and accordingly, interplant transfer of pollen by the wind is of little importance. Vigor is restored when two different varieties are cross-pollinated to produce the first generation (F1) progeny. A cross between two defined substantially homozygous cotton plant varieties always produces a uniform population of heterozygous hybrid cotton plants and such hybrid cotton plants are capable of being generated indefinitely from the corresponding variety cotton seed supply.
When two different, unrelated cotton parent plant varieties are crossed to produce an F1 hybrid, one parent variety is designated as the male, or pollen parent, and the other parent variety is designated as the female, or seed parent. Because cotton plants are capable of self-pollination, hybrid seed production requires elimination of or inactivation of pollen produced by the female parent to render the female parent plant or a specific flower male sterile. This serves to prevent the cotton plant variety designated as the female from self-pollinating. Different options exist for controlling male fertility in cotton plants such as physical emasculation, genetic male sterility, cytoplasmic male sterility and application of gametocides.
The development of new cotton plant varieties and hybrid cotton plants is a slow, costly interrelated process that requires the expertise of breeders and many other specialists. The development of new varieties and hybrid cotton plants in a cotton plant breeding program involves numerous steps, including: (1) selection of parent cotton plants (germplasm) for initial breeding crosses; (2) inbreeding of the selected plants from the breeding crosses for several generations to produce a series of lines, which individually breed true and are highly uniform; and (3) selecting superior new lines which will become cultivated varieties.
Cotton plant varieties and other sources of cotton germplasm are the foundation material for all cotton breeding programs. Despite the existence and availability of numerous cotton varieties and other source germplasm, a continuing need still exists for the development of improved germplasm because existing parent cotton varieties lose their commercial competitiveness over time. Embodiments of the present disclosure address this need by providing novel cotton varieties designated 99M-548, and an essentially derived transgenic progeny designated 548WRF-CB2-B, that possess broad adaptation and excellent yield stability in the early maturity cotton growing regions of the US; excellent fiber properties such as micronaire, length, strength (g/tex), and fiber uniformity; and a moderate level of resistance to root-knot nematodes. In addition to their value a varieties, 99M-548 and 548WRF-CB2-B contribute such characteristics to hybrids relative to other similar hybrids in the same maturity groups. To protect and to enhance yield production, trait technologies and seed treatment options provide additional crop plan flexibility and cost effective control against insects, weeds and diseases, thereby further enhancing the potential of these varieties and hybrids with 99M-548 or 548WRF-CB2-B as a parent.